British prison is a harrowing place to do time, but a surprisingly effective stage for a father-son reconciliation.

If you can’t understand half of what the characters say in “Starred Up,” not to worry. It’s an artistic choice on the part of director David Mackenzie (“Young Adam”), who clearly sees language as secondary in this brutal full-body immersion into the British prison system. There’s something primal about the way these inmates communicate, mixing their heavily accented British prison slang with bursts of spontaneous aggression. It’s a harrowing place to do time, but a surprisingly effective stage for a father-son reconciliation, and though the unintelligibility issue will severely limit its reach, this powerful dysfunctional-family saga is too well acted to go unreleased.

As prison films go, “Starred Up” hits all the usual bases with blistering naturalism: There’s brawling in the corridors, sexual tension in the showers and corruption among the guards. But the plot defies the genre, and its two central performances rank on par with those in 2009’s “A Prophet.” Instead of wanting to get out, 19-year-old Eric (“Skins” star Jack O’Connell in a career-making turn) desperately wants to stay behind bars, where he can be with his dad.

He’s a mixed-up young man, shuttled between state care facilities for most of his childhood after the death of his mother and incarceration of his father, Neville (Ben Mendelsohn, who betrays no trace of his Ozzie origins). “Starred Up” opens with Eric’s arrival at the same adult prison where Neville has spent the past 14 years, and the interactions that follow suggest that this severely compromised reunion may be the closest thing he’s ever had to a proper family environment.

The script doesn’t offer much in the way of backstory, though puzzle lovers will pick up the clues as the taut-jawed teen swiftly demonstrates his prison smarts, fashioning a weapon from a toothbrush and safety razor. Eric has clearly spent significant time behind bars already and must have angled to get himself “starred up” — a controversial practice by which the British prison system boosts especially dangerous juvenile offenders to the 21-and-over facility before their time.

What Eric doesn’t realize is that his status is a red flag to the warden (Sam Spruell) and a call to duty for volunteer therapist Oliver (Rupert Friend) determined to teach these tough cases anger management. What follows is a battle for Eric’s soul, as criminal elements try to corrupt him, Neville and Oliver attempt to reform him and the warden threatens to “warehouse him” — or keep such dangerous types off the street for good, even if it means staging what looks like suicide.

If that sounds like a lot of detail, the jaggedly assembled narrative demands a certain amount of explication that the brusque con talk — partly garbled by the characters, partly drowned out by badly mixed ambient sound — simply doesn’t provide. This film needs footnotes, not subtitles, though it seems unlikely that Mackenzie would agree to the sort of dub job that made “The Guard” more accessible abroad, if only because the helmer seems so committed to the reality inherent in Jonathan Asser’s script.

Despite its graphic violence and nudity, “Starred Up” never feels like an exploitation picture. At times, O’Connell calls to mind Tom Hardy’s turn in “Bronson” (especially in a scene where he strips down and greases up in order to take on a phalanx of prison guards), though the tone couldn’t be more different. For starters, the behavior onscreen feels too raw and immediate to register as “acting,” every twitch of it captured with handheld cameras and reinforced by a completely plausible supporting cast.

Mackenzie isn’t attempting to craft a larger-than-life antihero here, but delving into the sociology of this hellish subculture, where prisoners and staff alike coexist in this dehumanizing environment. The pic owes its believability to Asser, who served as a therapist similar to Oliver’s character, drawing from his experience to shape the world. Asser brings more than just realism, however, crafting the central father-son relationship on the foundation of classical Greek tragedy.

Eric’s agitation reads loud and clear from frame one, thanks to the startling unpredictability of O’Connell’s perf. The young thesp plays it in such a way that auds can never be certain whether he loves his father or wants to see him dead. The more he gets to know the guy, the less he admires — the polar opposite of the film, which will only get better upon closer inspection, as even the sharpest viewers are sure to miss a great deal the first time around. That’s why the father-son dynamic is so crucial to its success, lending a degree of universality to a story whose context is so uncompromisingly specific.